1. Field of the Invention
The present invention relates to a differential circuit with a dynamically and precisely controller terminator circuit, in particular, relates to a differential circuit used in an optical receiver.
2. Related Prior Art
Recent high speed communication system requests a termination resistor for an electronic circuit dynamically and precisely controlled. A United States Patent publication 20020145443 has disclosed an example of the termination circuit. FIG. 5 shows a conventional termination circuit 100 disclosed in the publication above, where the circuits 100 terminates the transmission line 101 and includes a resistor unit 102 and a reference unit 103. The resistor unit 102 includes a resistor 102a and an FET 102b connected in serial between the transmission line 101 and the ground. The FET 102b operates as a variable resistor. The reference unit 103 includes a resistor 103a and another FET 103b connected in serial to the resistor 103a, between the power supply line VDD and the ground, and also operates as a variable resistor. The terminator circuit 100 also provides a current source between the reference unit 103 and the ground. The operational amplifier 105 receives the node potential between the current source 104 and the reference unit 103, and provides the gate bias to the FET 103b so as to equalize the node potential above with the reference Vref. That is, the operational amplifier 105 varies the equivalent resistance of the FET 103b such that the voltage drop generated by the current from the current source 104 multiplied by the serial circuit of the FET 103b and the resistor 103a becomes equal to the difference between the power supply VDD and the reference VREF. This gate bias, determined by the feedback loop of the operational amplifier 105 and the reference unit 103, is also sent to the resistor unit 102 to dynamically vary the equivalent resistance of the FET 102b. Thus, the resistance of the resistor unit, which is a sum of the resistance of the resistor 102b and the equivalent resistance of the transistor 102b, may be stabilized in a preset value.
A PCT application, internationally published as WO95/24089, has also disclosed a termination circuit similar to those explained above.
However, the termination circuit 100 illustrated in FIG. 5 excludes a practical termination resistor in the resistor unit 102 from the feedback loop constituted including the operational amplifier 105. Consequently, when the resistance of the transistors, 102b and 103b, or the resistors, 102a and 103a, unbalance by various factors such as the process instability or the inherent temperature dependence of the device, or when the power supply VDD fluctuates, the accuracy of the termination resistance deteriorates.